Conventionally, commercially available servo amplifiers operate properly over a limited-load inductance range. Typically, the load inductance range may only be changed manually by substituting one or more inductance compensation values, e.g., one or more capacitors and/or resistors. The predetermined inductance compensation value allows the servo amplifier to operate over one inductance range at any given time. The above-described arrangement works well for motors which have a limited or constant load-inductance range. For example, the above-described arrangement works well for permanent magnet servo motors which operate in the linear characteristic region of the magnetic, e.g., in a region below magnetic saturation of the core. However, a problem arises in motors which operate both in a region where the cores are in magnetic saturation and where the cores are not in saturation or in motors where the inductance varies widely, e.g., in switch-reluctance motors.
Referring to FIGS. 5 and 6, it can be demonstrated that the step response of commercially available, single-load range, servo amplifiers is unsatisfactory when utilized with variable reluctance motors. Specifically, FIG. 5 shows the step response of a servo amplifier (i.e., a Model 432 DC Brush Servo Amplifier from Copley Controls Corporation, Westwood, Mass.) configured with an inductance compensation resistor of 200 k.OMEGA. and operated with a switched reluctance motor. Although the 200 k.OMEGA. inductance compensation resistor is suitable for operation of a motor in the unsaturated inductance condition, at current values above the saturation point, the low inductance is not compensated properly. In this configuration, as shown in FIG. 5, the voltage step of the amplifier input 100 results in a voltage step at the motor coil current 101 displaying oscillations.
Similarly, FIG. 6 shows the amplifier step response with the induction compensation resistor, e.g., an 18 k.OMEGA. resistor selected for operation with a saturated inductance. Although an 18 k.OMEGA. inductance compensation resistor eliminates the above described oscillation, the rise time response of the motor coil current 102 is adversely affected. For example, the rise time response of the servo amplifier, configured with the 18 k.OMEGA. inductance compensation resistor, is approximately twice the rise time associated with the amplifier configured with the 200 k.OMEGA. inductance compensation resistor. Although the oscillations may be prevented by reducing the value of the inductance compensation components, motor performance is adversely affected such that the maximum motor speed for a given motor force is reduced by a factor proportional to the increase in the rise time. Accordingly, conventional arrangements for inductance compensation of commercially available servo amplifiers are unsatisfactory.